In a nut shell, nVidia GeForce GTX 680 SLI scales really, really well. Overall perhaps a little bit less than AMD Radeon HD 7970 Crossfire, but in 5760x1080 it outperforms AMD's card. That's also the resolution where SLI truly comes into its own. If you only use one monitor, having two of these cards is overkill. But when gaming on three screens the second GTX 680 definitely improves performance, staying mostly above 60 fps without having to make (too many) concessions in terms of quality.
Combining three or four GTX 680s gives even better results, as you can see in the benchmarks, but in some cases it actually has the opposite effect. It's the usual story with triple- and quad-SLI and Crossfire configurations - it's primarily a treat for overclockers wanting to achieve the highest 3DMark scores. However, should you win the lottery or find that treasure in the back yard, playing around with four GTX 680s is great fun!